When a battery is brought into a highly charged state, a self-discharge is caused on the surface of an electrode, which causes a decomposition reaction of an electrolyte solution. An oxidation reaction is caused on the surface of a positive electrode, which generates an oxidized gas (for example, carbon dioxide). A reduction reaction is caused on the surface of a negative electrode, which generates a reduced gas (for example, hydrogen and carbon monoxide). When the gas is generated, the battery is swollen, which increases internal resistance. As a result, the capacity of the battery is disadvantageously decreased.